The present invention relates to orthopedic fastening systems and to mechanisms for securing and locking a linking or stabilizing element, such as a rod, to a bone screw having a slotted head that receives the rod therein. It also relates to structures or anchor assemblies having such a slotted or open head for receiving a rod, wherein the slotted portion extends from a hook, plate, bracket or positioning arm.
A number of such structures are known. Thus, for example, the widely used Harms T-plate used for stabilizing the cervical vertebrae has a projecting slotted bolt adapted to receive a rod or cable through the slot and clamp down by screwing a nut along the bolt to bear down against the rod. Several patents show holding structures for a fixation rod that are incorporated in the head of a screw, as in U.S. Pat. No. 5,672,176, or into a small offset plate which itself may be fastened to the bone, as shown in published International Application WO96/28105. Other systems involve hooks, transverse rod connectors, or tandem connectors. Various tools have been provided for these systems to enable the surgeon to bend and shape the rod to a desired contour in situ, to position the rod in the slot of a bolt or head, and to secure the rod in position.
Because the rod is the stabilizing member which provides a precise contour, spacing or connection between one or more vertebrae, bones or bone fragments, alignment is quite critical, and the ability to pass the rod through two or more connecting assemblies requires various actions to form and shape the rod, or align the receiving structures at defined positions or path before final clamping is effected. This may involve positioning and removing the rod several times to check and adjust the degree of alignment. Thus, it is, generally desirable to have a closure or secure locking mechanism that may be effected with simple installation steps.
One generally accepted locking mechanism simply involves an internally threaded locking nut that may be tightened down along the axis of the screw or slotted shaft, using a tool such as a socket wrench. Another commercial device employs a bayonet-mount cap that captures or is captured by the screw head, as in U.S. Pat. Nos. 5,346,493 and 5,257,993. Another system utilizes a cap element with a dovetail or dovetail channel that slides over the rod to close the top of the slot and wedge the rod firmly in position. This latter construction involves no rotation of threaded members, but has the disadvantage that a certain amount of unobstructed lateral space along the rod adjacent to the connection point is necessary for the sliding installation of the closure cap. Furthermore, the cap inserts or sliding wedge closures, while they eliminate the need for awkward screwing or rotational motion during installation, cannot be used with some existing reduction screws, translation hooks or other common hardware having lengthy protruding guide members, reduction tabs or the like. Moreover, the wedge/cap closures are a specialized component that may require the user to switch entirely over to a proprietary line of orthopaedic hardware if he is to utilize the full range of hook, tab, plate and screw fixation points that may be required in spinal surgery.
Accordingly, it would be desirable to provide a closure cap for a bone screw or similar anchor assembly to secure a fixation linkage such as a rod or cable.
It would further be desirable to provide a rod-securing closure mechanism for a screw or anchor that installs simply by a partial turn.
It would further be desirable to provide a rod-securing closure mechanism for a screw or anchor that requires minimal lateral clearance along the rod for its installation.
It would further be desirable to provide a rod securing cap or closure mechanism adaptable to diverse reduction screws.
One or more of these and other desirable traits are achieved in accordance with the present invention by a fixation assembly wherein a closure cap fits over an opening to close a rod-receiving slot of a fixation screw, hook, post or other anchor assembly, and capture the rod therein. A set screw threads through the cap and tightens against the rod to further clamp it in the assembly. In one embodiment, the closure cap extends over and around the head of the rod-receiving assembly, which may, for example include a slotted shaft, post or head, and the cap is adapted to lock together therewith by limited rotation. This may be accomplished in one embodiment construction by arranging the cap to have a set of sector rim protrusions positioned to fit through a corresponding set of protruding bosses or partial flange segments located on the head, and to rotate into opposition therewith for securing the cap onto the top of the fixation screw. The protrusions or flange segments are angled, along the radial direction, so that they bear against each other and jam when rotated, thus cannot slip out of engagement. The opposed segments tighten and lock the cap against the head when the cap is rotated through a partial turn of about twenty-three degrees of arc, like a flange-locking bayonet mount. The cap may have a rim that extends over the outside of the bolt head to engage external flange segments on the head. In a preferred embodiment, the mating portions may located internally in the head, with flange segments projecting radially inward from the perimeter, and the cap fitting between segments in the head of the bolt and locking with a twist-in motion to capture a rod in the opening. A hold-down set screw threads through a central opening in the cap and tightens down against the rod to clamp the rod firmly in place. The closing and clamping may each be effected by a driver tool that operates along the axis of the assembly and requires little or no side clearance to rotate either the cap or the set screw. In the case of the external, twist-on cap, the limited degree of rotation allows the cap to also include slots through the cap to accommodate reduction tabs extending upwardly from the underlying screw or anchor member. The quick-twist closure cap assemblies of the invention may be adapted to a wide range of screw, hook, eye, plate, connector and other anchor assemblies for rod, cable and other linking elements.